Shock absorption is an inherent function of the able-bodied locomotor system. Able-bodied ambulators are not typically consciously aware of shock attenuation, but lower-limb prostheses users are often acutely aware of the jarring impact forces that can occur when they take a step with their prosthesis. These impact forces are transmitted through the prosthesis to the residual limb, making walking uncomfortable and even painful. Components with reduced stiffness are commonly prescribed in lower-limb prostheses to change the mechanical response of the prosthesis to an applied load, providing shock absorption and reducing forces transmitted to the residual limb during gait. However, contrary to expectations, these components do not generally decrease ground reaction force (GRF) loading peaks during gait, a commonly used indicator of shock absorption. Previous reports of increased subject preference for reduced-stiffness components indicate that these components are influencing the limb-prosthesis system. However, this influence has not been documented consistently in any biomechanical gait parameter. Currently, the difference in prosthetic stiffness required to overcome the passive contributions in total limb stiffness and enact a change in the impact force profiles is unknown. Therefore, it is important to evaluate the effect of changes in prosthetic stiffness in vivo and in a controlled impact environment. Furthermore, only manufacturer-recommended stiffness levels for any of these reduced-stiffness components have been previously evaluated. It is also important to determine if prosthesis users actively modulate total limb stiffness during walking in response to changes in prosthesis stiffness, preserving some minimum magnitude of impact force when reduced-stiffness components are incorporated into their prostheses. The purpose of this proposed study is to systematically vary the stiffness of a transtibial prosthesis and measure the force response during in vivo impact loading and gait. Impact forces will be measured as the prosthetic stiffness is systematically varied while using a novel in vivo impact-testing protocol that reduces the ability of the prosthesis user to influence impact forces. These data will then be compared with a quantitative gait analysis performed over the same prosthetic stiffness levels. This proposed study will be the first to perform a systematic evaluation of the impact force-prosthetic stiffness relationship. Reduced prosthetic stiffness is hypothesized to decrease impact force magnitudes during impact testing. GRF loading peaks are not expected to change between prosthetic stiffness conditions, indicating an active compensation strategy by the prosthesis user during gait. The proposed study is intended to lay the groundwork for a more complete understanding of how prosthetic components function in vivo. First, it would corroborate subjective data that indicates that reduced-stiffness components are capable of overcoming the low stiffness of the soft tissue of the residual limb and influencing the limb-prosthesis system, even in the absence of documented biomechanical changes during walking. Furthermore, the anticipated results would indicate that subjects are able to accommodate to changes in prosthetic component stiffness, requiring further investigations to identify, better understand, and develop components that can capitalize upon these adaptive mechanisms. Finally, these results would inform clinical practice and prosthetic design by indicating which levels of reduced prosthetic stiffness are effective at reducing impact forces within the residual limb and prosthesis as a whole.